1. Field of the Invention
This invention relates to dimension measuring instruments, and more particularly to improvements in a dimension measuring instrument suitable for use in a limit gauge such as a test indicator, comprising an encoder for converting a displacement of a spindle into an electric signal and an analogue indication device for electrically indicating a measured value, which has been obtained through processing of an output from the encoder, by a pattern including a scale or scales, a pointer and the like.
2. Description of the Prior Art
There has heretofore been known a limit gauge such as a test indicator for detecting whether or not a dimension of a workpiece to be measured remains within allowable limits, e.g. a tolerance from a displacement in the measuring direction of a spindle when the tip end of the spindle is brought into contact with the workpiece to be measured.
As the limit gauges of the type described, there have been known a so-called mechanical limit gauge wherein a plurality of pairs of stationary contact points adjustable in their positions by use of adjusting screws and the like and movable contact points movable in operational association with a spindle or a pointer of a dial gauge are assembled into the dial gauge and whether or not the dimension of the workpiece is within the allowable limits is detected from the presence or absence of electric contact between the respective contact points, and a so-called electric limit gauge wherein a position of the spindle is caught as an electric signal, an indicated value of a measured value at a reference dimension can be set at zero, whether or not the dimension of the workpiece is within the tolerance is detected from a deviation from the reference dimension.
However, in the former mechanical limit gauge, the measured value is mechanically analogue-indicated by the pointer, whereby, although the sensible quantity of the measured value and the progress of approaching the critical value or the like have been intuitively sensed, high skill level has been needed for accurately reading the measured value. Furthermore, a scale plate cannot be easily changed, and it has been difficult to change a range of measuring. Further, since the pointer has been adapted to move in mechanical association with a displacement of the spindle, such drawbacks have been presented that when the dimension of the workpiece to be measured has been unusually large, or when an impact or the like is applied to the spindle, the pointer has made a scaleover so that a measured value cannot be read, or the analogue indication device has come to be out of order.
On the other hand, in the latter electric limit gauge, a numerical value as the result of measurement is electrically and digitally indicated, whereby, although the reading of the measured value has been facilitated to a considerable extent, such a disadvantage has been presented that the sensible quantity of the measured value or the progress of approaching the critical value cannot be intuitively sensed.
To obviate the above-described disadvantage of the electric limit gauge, the present applicant has proposed in Japanese Patent Laid-Open No. 19503/83 a dimension measuring instrument for measuring a length and the like of a workpiece to be measured from a displacement of a spindle when the tip end of the spindle is brought into contact with the workpiece, wherein there is provided an encoder for converting the displacement of the spindle into an electric signal, a signal processing circuit for processing an output from the encoder to obtain a signal suitable for the indication, a digital indication device for electrically indicating a numerical value and an analogue indication device for electrically indicating a pattern including a scale within a measuring range, a pointer and the like, and a measured value such as a length or the like of the workpiece to be measured, numerical values of the scale within the measuring range, a critical value or setting data as being set values, a result of determination as to whether the measured value passes or fails the critical value, the maximum or minimum value of the measured value or measured data as being an error to the set value are indicated by the digital indication device and the analogue indication device at the same time.
The dimension measuring instrument of the type described has such characteristic features that the measured value can be easily read, the sensible quantity of the measured value or the progress of approaching the critical value can be intuitively sensed, and further, the setting data and the measured data can be indicated at the same time.
However, in the conventional electric analogue indication device, since the analogue indication has been performed at a uniform scale interval similarly to the digital indication device, if the range of indicated dimension is expanded, such disadvantages have been presented that the scale interval is enlarged and the scale spacing is decreased, whereby it becomes difficult to read. Whereas, if the scale interval is decreased so as to make reading easy, then the range of dimension indication becomes narrow, whereby the scaleovers occur frequently.
In contrast thereto, in the practical measurement, on one hand, it is necessary to read the dimension within the tolerance with high accuracy, on the other hand, an operator observes the indication of the scaleover through the dimension out of the tolerance and moreover desires to learn a quantitative rough estimate of the dimension therethrough in many cases. Particularly, in the case where the results of measurement are caught as a group, the standard deviation, the process capability and the like are sought and the quality control is effected, and further, when a production machine is regulated on the basis of these data, the quantitative rough estimate is needed for predicting the tendency. More specifically, necessity has been voiced for learning a variation in the dimension of the workpiece to be measured assuming a normal distribution-like distribution over the total area. However, it has been difficult to satisfy such a necessity with the conventional analogue indication device in which the scale interval is made uniform.